A Wireless Sensor/Actuator Network (WSAN) is a group of sensors that gather information about their environment and communicate wirelessly with actuators that interact with the environment. In many scenarios the WSAN devices are power constrained, which imposes restrictions on wireless communication subsystem in such devices. There is a general incompatibility between design flexibility and a low-power consumption requirement for implementation of Medium Access Control/Link Layer (MAC/LL), which is Layer 2 of a protocol stack.
Wireless connectivity protocols with scheduled channel access are prevalent for WSAN in wearable devices and the Internet of Things (IoT). In a WSAN, slave (sensor or actuator) nodes have low wireless activity duty cycles; as an example, in some scenarios fast connections present duty cycles of approximately 2%, while slow connections present duty cycles of approximately 0.02%. Therefore, wireless idle-time power optimization is a significant Key Performance Indicator (KPI) factor. This optimization is inherent in MAC/LL design because real-time mechanisms that handle the air interface protocol in wireless connectivity pertain to the MAC/LL. Upper layers of the protocol stack are decoupled from air interface timing, and coupled to host application execution time. Therefore MAC/LL design is a main focus for optimizations targeting wireless idle-time power consumption.
In order to reduce the Time-To-Market (TTM) for WSAN devices, conventional systems use programmable Central Processing Units (CPUs) (e.g., General Purpose Processor (GPP), Micro Controller Unit (MCU), etc.), and implement most of the MAC/LL protocol as software relying on a real-time control environment, such as Real Time Operating System (RTOS). In such a system, the idle-time power consumption is dominated by the CPU dynamic power consumption (CPU is processing while air interface is idle) and by leakage power from persistent subsystems (such as retention memory) while the CPU is idle.
While the main power consumption in these conventional systems during wireless activity periods comes from the physical layer (PHY), during wireless idle periods the PHY is off, and the main power consumption is by the MAC/LL, that is, from CPU execution and the power leakage due to the program/data that needs to be maintained in memory. For protocols with low wireless activity duty cycles, the wireless idle period power consumption contributes to the total power budget significantly.